It's not always to "be popular", the biggest reason English is the main language of Tapastic is simply because . . . well, English is the most universally accepted language. Sure, Mandarin is technically the language that has the most speakers based on population, and yeah, there are lots of people out there who have different native tongues, but those people also usually speak English as their second language, and when it's the most universally understood and acceptable for, well, EVERYTHING, it's just common sense to do a comic in English. It's not even about "popularity", it's about accessibility. Plus the website is built in English so of course it's going to be the "main" language of the comics on the website.
Also, there are plenty of non-English comics here, but most of them are offered as a second translation. If you check out the panel below the comic's thumbnail (near the subscribe button) you'll see a yellow button (between the RSS feed button and the Share Comic button). Click on that, and you'll see a drop-down menu available of all available translated languages for that comic, from Korean to Spanish and all other languages that are available to choose from here on the website. Note that it's not on all comics, only comics that have a second language registered for translated pages (turns out Big Jo has translated pages for Portuguese, for example). English is just available as the recommended "default option" (although there are a few comics that are ONLY one other language, and don't offer an English translation, I've seen a Spanish version of Fisheye Placebo floating around for ex.), and makes sense considering the company that runs Tapastic is based in the US.
So there are lots of comics here that don't use English, you just need to put more work into finding them and not blame the language itself for being as internationally accepted as it is 
